The present invention relates to an image reading apparatus for converting optical image information into an electric signal, and more specifically relates to a line sensor of the multi-chip type comprised of photoelectric conversion elements and a scanning circuit, which are arranged linearly and integrally.
FIG. 3 shows the conventional image reading apparatus constructed such that photoelectric conversion elements 1.sub.l -1.sub.n are connected to a common line 4 through a plurality of first switching elements 2.sub.l -2.sub.n, respectively. An image signal is sequentially outputted from an image signal output terminal 18 through a third switching element 14. FIG. 4 is a timing chart showing the operation of the FIG. 3 image reading apparatus.
However, in the conventional image reading apparatus, since a load capacitance and resistance is coupled to the common line 4, the image signal SIG produced from the respective photoelectric conversion elements 1.sub.l -1.sub.n rises at a certain time constant determined by the load capacitance and resistance. In view of this, when increasing a frequency of a clock pulse CK for use in driving the image reading apparatus, the amplitude of the obtained image signal SIG decreases to thereby cause reduction of the SN ratio.